In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. Measurements in a wellbore, also referred to as a borehole, are typically performed to attain this understanding. The usefulness of such measurements may be related to the precision or quality of the information derived from such measurements.
Many measurement tools need calibration, including an array laterolog tool (LAT). Calibration of the LAT can be performed with the LAT connected to a calibration box (CB). The calibration box consists of a network of resistors, which simulates electrical loading of the LAT, corresponding to various formations. The resistors electrically simulate the corresponding borehole-formation impedance between all electrode pairs. Usually several different calibration boxes for different borehole-formation conditions should be designed to implement shop calibration of the LAT. Ideally, the calibration boxes should produce substantially similar results with the results obtained by using formations, where the results are apparent resistivity. Apparent resistivity is the resistivity of an electrically isotropic and homogenous region that corresponds to a measured relationship between applied current and potential difference for a given configuration of electrodes such that the apparent resistivity can be provided as a product of measured resistance and a geometric factor.
Once the LAT is connected to the calibration box, unfocussed voltages and currents (V(u) and I(u)) are measured. Unfocused measurements can be focused by using a software focusing code by which focused measurements (V(f) and I(f)) are obtained. By using the focused measurements calibration is performed. Such precision or quality may depend on calibration of the measurement tools.